Preventing damage or discomfort resultant from impacts or vibration is a ubiquitous problem in our society and there is an increasing need for advanced energy absorbing materials. Cellular materials, e.g. expanded polystyrene (EPS) are typically used for these purposes, depending the choice on the application itself. The majority of these materials deform by crushing, developing a permanent deformation and limiting their use to just one.
In the last decade Shear Thickening Fluids (STFs), which are a particular type of complex fluids, have attracted the attention of the industry for the fabrication of passive dissipative devices, such as vibration absorbers and ballistic and stab resistant fabric composites, due to their viscosity increase with the applied shear stress over a critical value. Additionally, these liquids do not require an external activation mechanism, as magneto-rheological or electro-rheological fluids do, since they just activate under stress. Moreover, the increase in the viscosity can be tailored for the specific application by choosing properly the components of the STF. All this has led to a considerable interest in incorporating STFs into other materials in order to obtain energy absorbing composites possessing a combination of their best properties/characteristics. In some prior approaches, STFs were encapsulated into sealed bags, with syntactic glass beads for weight reduction (US 2005/0266748 A1); or incorporated into solid phase elastomers (US 2006/0234572 A1).
As the rheological response of STFs is greatly affected by the deformation rates at which it is undergone, what depend directly on the local geometry and the applied forces, other inventions propose energy-absorbing composites with STFs based on the interaction between the fluid and the geometry which confines it. Fluid-impregnated material consisting of a porous interconnected network of solid material forming edges and faces of cells, preferably an open-cell reticulated or partially closed-cell foam, or formed from fibers or other cellular solids (U.S. Pat. No. 8,091,692 B2), where the tortuosity of the passageways subjects the STF to a complex flow under confinement. Thus, the addition of STFs to the porous media increases their energy absorption capabilities, due to the contribution of the viscous work done by expelling the fluid from inside the cells of the foam, which is added to the energy dissipated due to the elastic, plastic, and buckling modes that occur during compression of the scaffold material. Alternatively to the impregnated foams, other approach consisted of two outer layers containing reservoirs or chambers of STF separated by a shear layer including a lattice structure defining straight shear paths between the first and second outer layers (US 2014/0259326 A1). Finally, another invention comprised two pieces of solid material disposed in a superimposed relationship, configured so as to define a plurality of chambers there between as well as a plurality of fluid flow channels, which is in fluid communication with two of the chambers; in this way, when the applied impact exceeds the predetermined sealing level, the seals close off the chambers from the fluid channels converting the material to an closed cell structure (WO 98/23179 A1).
It has been reported recently that STFs enhance their shear thickening behaviour when sheared under confinement, i.e. when flowing through tiny channels. Microfluidics is the science and technology of devices and methods that process, control or manipulate very small amounts of fluid by using channels with characteristic length-scales less than a millimeter. Due to the numerous advantages, microfluidics have stimulated remarkable interest and unravelled an extensive range of applications, from biotechnology to enhanced oil recovery.
These facts are disclosed in order to illustrate the technical problem addressed by the present disclosure.
General Description
The present disclosure relates to energy absorbing composites comprising a resilient solid material, a numerically optimized closed microfluidic networks, and a complex fluid, either VEFs or STFs. The present disclosure is able to absorb or dissipate energy resulting from impacts or vibrations without the need to rely on magneto-rheological or electro-rheological fluids.
Energy absorbing composites are made up of distinct components, elements, or parts, and combining their essential or typical with the aim of absorbing or dissipating external kinetic energies in larger amounts than their separate identities. In the particular case of the present disclosure one component consists of a resilient solid material and the other component consists of a complex fluid, embedded into the previous one by means of optimized microfluidic networks.
Resilient solid material may be, for example, microagglomerated cork, expanded polystyrene (EPS), expanded polypropylene (EPP), ethylene vinyl acetate (EVA), etc.
Cork is a natural cellular material that is recently being considered for its use in lightweight structural and energy-absorbing applications due to cork is capable of absorbing considerable amounts of energy with almost total reversibility useful for the repeated absorption of impact energy. Micro-agglomerated cork is produced out of waste cork coming from the production of stoppers. Cork granules with particle size below 1 mm are bonded to each other either by activating their natural resins (pure agglomerated cork) or by coating the granules with a thin layer of an additional adhesive agent (compound agglomerated cork). Thus micro-agglomerated cork exhibits more homogeneous properties and greater variety of geometries than its natural form.
Expanded polystyrene (EPS) is a rigid and tough, closed-cell foam. It is usually white and made of pre-expanded polystyrene beads. EPS is used for many applications e.g. trays, plates, bowls and fish boxes. Other uses include molded sheets for building insulation and packing material (“peanuts”) for cushioning fragile items inside boxes.
Expanded polypropylene (EPP) is a highly versatile closed-cell bead foam made of polypropylene. EPP has very good impact characteristics due to its low stiffness; this allows EPP to resume its shape after impacts.
Ethylene-vinyl acetate (EVA), also known as poly(ethylene-vinyl acetate) (PEVA), is the copolymer of ethylene and vinyl acetate. Ethylene vinyl acetate (EVA) is the copolymer of ethylene and vinyl acetate. It's an extremely elastic material that can be sintered to form a porous material similar to rubber, yet with excellent toughness.
A complex fluid (also known as non-Newtonian fluid) is a fluid that exhibits a stress-strain rate relationship that does not follow the linear Newton's law of viscosity. Complex fluids do also not follow Hooke's law of elasticity, the relationship between stress and deformation that is used for elastic materials.
A viscoelastic fluid (VEF) is a particular type of complex fluid VEFs which viscosity typically diminishes under shear (shear thinning behaviour); but when the applied load is removed, the stress inside the VEF does not instantly vanish and the internal molecular configuration of the fluid can sustain stress for some time (relaxation time).
A shear thickening fluid (STF) is a particular type of complex fluid with increasing viscosity and normal force when is undergone to a shear stress over a critical value. This critical value depends on the particular formulation of the fluid. If the STF is made of colloidal particles, then the onset of the shear thickening fluid will depend on the particle size, particle shape and volume concentration.
Microfluidic network is meant as a network of channels with characteristic length-scales less than one millimeter.
The use of microfluidic network for embedding the complex fluids into the resilient solid material introduces several major advantages with regards to the strategies implemented in the art:                reduced amount of fluid, which is crucial for applications in which lightweight is crucial, for example sporting equipment such as helmets, helmet liners, ballistic equipment, clothing, cushioning bodies;        enhanced rheological response of the STFs and VEFs;        optimized geometry of the fluid passageways, in particular microchannels;        allows to obtain an optimized energy absorbing composite with the required energy absorbing properties for any particular application, saving in this way weight and volume with regards to other current technical solutions.        
The geometry of the energy absorbing composites can be either 2D or 3D, depending on the preferred fabrication technique and the scaffold material. Thus, the simplest embodiment comprises of a laminar sheet of a resilient solid material, either engraved/carved/stamped/incised with an optimized network of microchannels, filled with a complex fluid and closed tightly with another sheet of solid material, as disclosed in FIG. 1. Thus the mechanical properties of the energy absorbing composite result from the combination of the mechanical properties of the solid material and the enhanced response of the complex fluid flowing through the network of microchannels, as well as the fluid-structure interaction.
Microfluidics is particularly interesting for this disclosure because of the reduced amounts of fluid sample needed and the possibility of producing highly integrated devices able to mimic porous media. Moreover, the geometric features microchannels can be numerically optimized in order to get the intended flow characteristics. Additionally, non-Newtonian fluids in general and viscoelastic fluids (VEFs) in particular, when flowing through microchannels increase significantly the relevance of fluid elasticity and, therefore, the flow resistance can be significantly different from those of their Newtonian counterparts at low Reynolds number, particularly if the microchannels are designed especifically for that purpose, as it is the case of the microfluidic rectifiers. This latter feature opens the door to the use of VEFs to develop energy absorbing composites, which is an advancement in the art.
The prior art presents several disadvantages when comparing with the present disclosure as:                none of the strategies for fabricating STF-composites implemented in the prior art takes advantage of the fact that confinement enhances the response of STFs;        none of the already disclosed prior art considers the possibility of using VEFs for the development of energy absorbing composites;        none of prior art considers the possibility of using computational techniques to optimize the fluid passageways, in particular microchannels, in order to maximize the safety and comfort of the composite and control the amount of energy absorbed or dissipated by the composite.        
Therefore a new line of technology is required to produce customized energy absorbing materials reinforced with complex fluids.
There is disclosed herein a technology for developing optimal and customized energy absorbing composites based on the combination of the mechanical properties of a resilient solid material, in particular microagglomerated cork, EPS, EPP, EVA and the rheological properties of complex fluids, in particular VEFs or STFs, embedded in the solid material by means of a numerically optimized microfluidic network.
These energy absorbing composites may be employed as a component of helmet liner, cushioning body or other such protective structure to prevent damage or discomfort from external mechanical dynamics, such as impacts or vibrations. Additionally, this technology may allow certain resilient solid materials, in particular microagglomerated cork, to accomplish the standards of certain applications (like helmet liner for motorcyclists, EN1621-1 and EN1621-2).
In an embodiment, the energy absorbing material now disclosed may comprise at least one support layer.
In an embodiment, the energy absorbing material may comprise two laminar sheets, or two layers, of a resilient solid material. The first layer may be a support layer and it may be embedded with an optimized network of microchannels and filled with a complex fluid, while the second layer may be closed tightly with the support layer.
In an embodiment, the geometry of the microfluidic network may result from a numerical optimization process, which will take into consideration the dynamics of the external mechanical input, either impact or vibration, the rheology of the complex fluid and the mechanics of the solid material (fluid-structure interactions).
In an embodiment, there is no need for having chambers or reservoirs of the fluid in the network and there is also no need for having parts disposed in a superposed relationship, like in the disclosure disclosed at US 2014/0259326 A1 and WO 98/23179 A1, respectively.
In an embodiment, the technology to embed the microfluidic channel on the solid material may depend on the nature of the solid material selected, in particular laser engraving may be used on microagglomerated cork; while micro-milling may be more adequate for EPS or EPP.
In an embodiment, the filling process of the microfluidic network with the complex fluid will also depend on the rheological properties of the fluid, in particular in the case of VEFs, the two sheets of solid material may be glued before filling the microfluidic network, and then the fluid can be infused into the microchannels.
In an embodiment, the filling process of the microfluidic network with the complex fluid will also depend on the rheological properties of the fluid, in particular in the case of STFs, the fluid can be spread onto the sheet embedded with the microfluidic network, filling the microchannels and then, bonded to the other sheet of solid material.
In an embodiment, the support layer comprises a closed cell foam structure.
In an embodiment, when the composite is subjected to an impact above a certain level, the outer sheet of solid material is deformed, part of the microfluidic channels are compressed and the fluid expelled out; then, the fluid is accelerated and the shear thickening behaviour (in the case of using a STF) or the elastic instabilities (in the case of using a VEF) will be triggered; and the energy of the impact will be absorbed and dissipated to the maximum by the combined effect of the solid material, the complex fluid and the fluid-structure interaction.
In an embodiment, then the composite is subjected to a vibration above a critical level, the complex fluid will activate, either shear thickening behaviour or elastic instabilities for STFs and VEFs, respectively, and, subsequently, part of the energy from the vibration will be dissipated by the combined effect of the complex fluid (sort of viscous damping), the solid material (hysteric damping) and the fluid-structure interaction.
The present disclosure also relates to a composite layer material for dampening external dynamic load comprising at least a support layer of a resilient material, said support layer having recessed fluid-tight microchannels comprising a fluid, wherein the microchannel section and fluid viscosity is such to dampen the external dynamic load by the constricted fluid flow through said microchannels.
In an embodiment, the fluid is a shear thickening fluid, a viscoelastic fluid, and combinations thereof.
In an embodiment, the external dynamic loads are impact and/or vibrations.
In an embodiment, the microchannels may be interconnected.
In an embodiment, the support layer comprises recessed fluid-tight pockets interconnected with said microchannels.
In an embodiment, the composite may further comprise a second layer placed over said microchannel and/or pockets for the retention of said fluid.
In an embodiment, the microchannels may extend in two planar directions of the support layer.
In an embodiment, the composite may comprise a plurality of support layers, in particular 2, 3, 4, 5 or more support layers.
In an embodiment, the microchannels may be engraved microchannels, carved microchannels or stamped microchannels.
In an embodiment, the microchannels and pockets may comprise a depth between 0.01 a 10 mm, preferably between 0.1 a 1 mm.
In an embodiment, the microchannels may comprise a width between 0.01 a 10 mm, preferably between 0.1 a 5 mm.
In an embodiment, the viscosity of the fluid may be between 10−3 a 104 Pa s at 20° C.; and the density of the fluid may be between 800 a 2000 kg/m3 at 20° C.
In an embodiment, the shear thickening fluid may be selected from a list consisting of concentrated dispersions of: corn starch, precipitated calcium carbonate, aerosil, fumed silica, silica and mixtures thereof, and others.
In an embodiment, the viscoelastic fluid may be selected from a list consisting of concentrated solutions of: polyacrylamide, polyethylene oxide, polyisobutylene, mixtures thereof, and others.
In an embodiment, the composite comprises an impermeable resilient solid material, which may be selected from a list consisting of cork, expanded polystyrene, expanded polypropylene, ethylene vinyl acetate, combinations thereof, and others.
In an embodiment, the resilient material may be an agglomerated material, in particular agglomerated cork.
In an embodiment, the agglomerated cork may comprise a granule size between 0.1 a 5 mm.
In an embodiment, the support layer of the composite may comprise a thickness between 1 a 10 mm, preferably 2 a 5 mm.
In an embodiment, the second layer of the composite may comprise a thickness between 0.1 a 10 mm, preferably 1 a 2 mm.
In an embodiment, the second layer of the composite may further comprise an adhesive.
The present disclosure also relates to shin guards, elbow guards, helmets, knee pads, body armours, insoles, anti-vibration pads, anti-vibration gloves, anti-vibration mats, anti-vibration mounts, acoustic isolator or any other vibration or shock/impact isolator comprising the composite layer material previously described.
Throughout the description and claims the word “comprise” and variations of the word, are not intended to exclude other technical features, additives, components, or steps. Additional objects, advantages and features of the present subject-matter will become apparent to those skilled in the art upon examination of the description or may be learned by practice of the invention. The following examples and drawings are provided by way of illustration, and they are not intended to be limiting of the present subject-matter. Furthermore, the present subject-matter covers all possible combinations of particular and preferred embodiments described herein.